After a long day of searching for yooperlite, it can be hard to put the blacklight flashlight away, and you may find yourself running around a dark house shining ultraviolet (UV) light onto random objects, making surprise observations.

Armed with a $10 UV flashlight acquired from a local hardware store*, I was quite surprised to see a strong response coming from the bunch of bananas. In particular, the part that was glowing brightest was the peel right around each brown spot.  Under normal light, that peel in these areas did not appear different from the rest of the yellow peel at all, but these areas fluoresce brightly under the UV light.

Called “blue halos of death” in papers by Moser et al. and  Müller and Kräutler, these rings are only associated with age spots (not bruises) where the cells have died. These authors concluded that the glowing rings were caused by an accumulation of chlorophyll degradation products. The area around the dark spot that glows brightly still contains living cells, but they have started to degrade due to aging processes.

Interestingly, the brown aging spots on bananas are centered on stomata and the Moser et al. paper has great scanning electron microscope images showing that (see their fig 5). While the “blue halos of death” were very noticeable in my case, and matched the images in the papers mentioned previously, a different author (Tiessen) didn’t see these halos in his experiments at all!

While the blue halos were what caught my eye, the entire banana itself fluoresces when ripe but not when green/unripe. This is apparently because the abundant chlorophyll in green bananas absorbs most of the fluoresced blue light—meaning the chlorophyll effectively hides the fluorescent quality of the banana, so green bananas don’t glow but yellow ones do (Tiessen, 2018)!

*Equipment Note

I would recommend spending a bit more on the UV flashlight, to hopefully get something higher quality, as the $10 version has something loose and requires constant smacking/shaking/physical coercion to get it to be full brightness and seems to randomly change brightness while in use.

Additional Reading

1. In Defense of Plants: Fluorescent Bananas. Aug 13, 2018.
2. Moser, S., et al. Fluorescent chlorophyll catabolites in bananas light up blue halos of cell death. Proceedings of the National Academy of Sciences Sep 2009, 106 (37) 15538-15543; doi: 10.1073/pnas.0908060106.
3. Müller T, Kräutler B. Chlorophyll Breakdown as Seen in Bananas: Sign of Aging and Ripening – A Mini-Review. Gerontology 2011;57:521-527. doi: 10.1159/000321877.
4. Science and Plants for Schools “Why do bananas fluoresce? An unexpected view of chlorophyll” (teaching resource).
5. Tiessen, A. The fluorescent blue glow of banana fruits is not due to symplasmic plastidial catabolism but arises from insoluble phenols estherified to the cell wall. Plant Science. Volume 275, October 2018, Pages 75-83. doi: 10.1016/j.plantsci.2018.07.006 (abstract available, but article behind paywall – see reference 6).
6. Tiessen, A. Fluorescent Blue Glowing Bananas: A Mystery Revisited. Science Trends. November 13, 2018.